iOS NSUserDefaults Watching the change of values for a single key

Swift Version:

func setUserDefaultsListener(){
    NSUserDefaults.standardUserDefaults().addObserver(self, forKeyPath: "keyPath", options: NSKeyValueObservingOptions.New, context: nil)
}

override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [String : AnyObject]?, context: UnsafeMutablePointer<Void>) {
    if keyPath == "keyPath" {
        //Do something
    }
}

deinit {
    NSUserDefaults.standardUserDefaults().removeObserver(self, forKeyPath: "keyPath")
}

Swift 3 Version:

override func viewDidLoad() {
    super.viewDidLoad()
    UserDefaults.standard.addObserver(self, forKeyPath: "keyPath", options: .new, context: nil)
}

override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    if keyPath == "keyPath" {
        //Do something
    }
}

deinit {
    UserDefaults.standard.removeObserver(self, forKeyPath: "keyPath")
}

Edit: viewDidUnload is now deprecated, use dealloc instead to removeObserver

This should work perfectly, I have just tested here.

- (void)viewDidLoad
{
    [super viewDidLoad];

    [[NSUserDefaults standardUserDefaults] addObserver:self
                                            forKeyPath:@"SomeKey"
                                               options:NSKeyValueObservingOptionNew
                                               context:NULL];
    // Testing...
    NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults]; 
    [defaults setObject:@"test" forKey:@"SomeKey"];
    [defaults synchronize];
}

- (void)viewDidUnload
{
    [super viewDidUnload];

    [[NSUserDefaults standardUserDefaults] removeObserver:self forKeyPath:@"SomeKey"];
}

- (void)observeValueForKeyPath:(NSString *) keyPath ofObject:(id) object change:(NSDictionary *) change context:(void *) context
{
    if([keyPath isEqual:@"SomeKey"])
    {
       NSLog(@"SomeKey change: %@", change);
    }
}

Things you could test.

• Put a break point in viewDidUnload and make sure the view isn't disappearing on you (since you are changing SomeKey from another viewController) If this is the case then maybe move your register/de-register code into init/dealloc depending on how your VC works.

• Use explicit KeyPath like @"SomeKey" not a substitution like SOME_NSSTRING_VARIABLE